This is the Missing Sector Orb Web spider, Zygiella x-notata. This is a very common and widespread species and loves to make its web over windows and uses the corners if the frame and the sill for its refuge. I doubt there is a home in the land that does not have this species in residence.

It is also the species most commonly mistaken for the Noble False Widow, Steatoda nobilis. Zygiella spp. are completely harmless and through the year they will catch a good deal of insects and other invertebrates that are considered pests. They are also one of the few species that happily feed through the winter given available winter insects.